1. Filed of the Technology
The technology presented herein relates to an antireflective member with good antireflection performance and to an optical element and a display device including such an antireflective member. The present technology also relates to a method of making a stamper (which will also be referred to herein as a “mold” or a “die”), a method of making an antireflective member using the stamper, and to an antireflective member.
2. Description of the Related Art
Display devices for use in TVs, cellphones and so on and optical elements such as camera lenses usually adopt antireflection technology to cut down the surface reflection and transmit the incoming light as much as possible. This is because when light passes an interface between two media with mutually different refractive indices (e.g., when light enters an interface between the air and glass), the percentage of the light transmitted decreases due to Fresnel reflection and other types of reflection, thus affecting the visibility.
As a method of applying the antireflection technology, an antireflective multilayer coating, which is a stack of a number of thin films made of inorganic particles of silica or organic particles of acrylic resin, for example, may be deposited on the surface of a substrate. However, such an antireflective multilayer coating is often deposited by an evaporation process, for example, which means it takes a lot of time and cost to finish such a process. Particularly under an environment where the ambient light is very intense, the antireflection performance needs to be further enhanced. In that case, the number of thin films stacked in the antireflective multilayer coating should be increased to further raise the cost eventually. Besides, since the antireflective multilayer coating utilizes the interference phenomenon of light, its antireflection performance heavily depends on the angle of incidence and wavelength of incoming light. That is why if the angle of incidence or wavelength of incoming light were outside of its preset range, then the antireflection performance would decline significantly.
The antireflection technology may also be applied by forming a fine uneven surface pattern, of which the periods are controlled to be equal to or shorter than the wavelength of visible radiation, on the surface of a substrate (see Patent Documents Nos. 1 to 5). This method utilizes the principle of a so-called “motheye” structure, in which the refractive indices with respect to light that has entered a substrate are changed continuously in the depth direction of the unevenness from the refractive index of the incidence medium to that of the substrate, thereby cutting down reflections in the target wavelength range. As the uneven surface patterns, cones, pyramids and other shapes with a sharpened top are shown as examples (see Patent Documents Nos. 3 to 5).
Hereinafter, it will be described with reference to FIGS. 11(a) and 11(b) exactly how the antireflectivity is realized by forming such a fine uneven pattern. FIG. 11(a) is a cross-sectional view schematically illustrating a substrate on which an uneven surface pattern with a rectangular cross section has been formed. FIG. 11(b) is a cross-sectional view schematically illustrating a substrate on which an uneven surface pattern with a triangular cross section has been formed.
First, referring to FIG. 11(a), the substrate 1 including such a rectangular unevenness 2 thereon will have the same function as a substrate including a single-layer thin film thereon.
The antireflectivity of a single-layer thin film will be described briefly. Suppose visible radiation with a wavelength λ is going to be incident on a glass substrate on which a single-layer thin film with a thickness d is deposited. To reduce the reflection of a perpendicularly incoming light ray (with an angle of incidence of zero degrees) to zero, a single-layer thin film that causes interference between the light reflected from the surface of the thin film and the light reflected from the thin film-glass substrate interface needs to be formed such that these two types of reflected light cancel each other. More specifically, the thickness d and the refractive index n of the single-layer thin film may be defined as d=λ/4n and n=(ni×ns)1/2, where ni is the refractive index of the air and ns is the refractive index of glass. Since the refractive index ni of the air is 1.0 and the refractive index ns of glass is approximately 1.5, the refractive index n of the single-layer thin film calculates approximately 1.22. Therefore, theoretically speaking, if a single-layer thin film, of which the thickness is equal to a quarter wavelength and which has a refractive index of about 1.22, is formed on the surface of a glass substrate, the reflection should be eliminated totally. Actually, however, organic materials for use to make thin films have as high refractive indices as about 1.5 or more and even inorganic materials, of which the refractive indices are smaller than those of organic materials, have refractive indices of approximately 1.3. That is why actually it is impossible to make such a substrate.
Next, the antireflection function caused by the very small rectangular unevenness 2 such as that shown in FIG. 11(a) will be described. In that case, the same effects as those achieved by forming a single-layer thin film with a refractive index of about 1.22 can be achieved, and the reflection can be totally eliminated, by optimizing the period of the unevenness. Nevertheless, as in the situation where the single-layer thin film is used, it is still difficult to realize the antireflection function in a broad wavelength range or with little dependence on the angle of incidence.
On the other hand, if the unevenness has a triangular cross section as shown in FIG. 11(b), the refractive indices with respect to the light that has entered the substrate change in the depth direction of the unevenness. As a result, the surface reflection decreases. Besides, if an uneven surface pattern such as that shown in FIG. 11(b) is formed, such an uneven surface pattern not just realizes the antireflection function in a broader wavelength range with much less dependence on the angle of incidence than the antireflective multilayer coating described above, but also is applicable to a lot of materials and can be arranged directly on a substrate. Consequently, a high-performance antireflective member can be provided at a reduced cost.
The motheye structure is usually formed by transferring the inverted fine uneven surface pattern from the surface of a stamper (which is a mold or a die) onto a light transmitting resin by an embossing process, an injection molding process or a casting process, for example.
In the prior art, the stamper is ordinarily made by a laser interference exposure process or an electron beam (EB) exposure process. According to any of these methods, however, it is totally impossible, or at least very difficult, to make a stamper with a large area.
Meanwhile, Patent Document No. 6 discloses a method of mass-producing stampers at a reduced cost by using anodized porous alumina, which is obtained by anodizing aluminum.
The anodized porous alumina, obtained by anodizing aluminum, will be described briefly here. A method of making a porous structure by anodization has attracted attraction as a simple method for making nanometer-scale columnar micropores that are arranged regularly. Specifically, a base material is immersed in either an acidic electrolytic solution such as sulfuric acid, oxalic acid or phosphoric acid or an alkaline electrolytic solution and a voltage is applied thereto using the base material as an anode. Then, oxidation and dissolution will advance in parallel on the surface of the base material. In this manner, an oxidized coating with micropores can be formed on the surface. Those columnar micropores will be aligned perpendicularly to the oxide film and exhibit self-organized regularity under certain conditions (including voltage, type of the electrolytic solution and temperature). Thus, this anodized porous alumina is expected to be applied to various functional materials.
As schematically illustrated in FIG. 12, the anodized porous alumina layer 10 consists of cells 16 of a predetermined size, each including a micropore 12 and a barrier layer 14. If the porous alumina layer is made under particular conditions, each cell 16 thereof has a substantially square hexagonal shape when viewed perpendicularly to the film plane. Also, when viewed perpendicularly to the film plane, the cells 16 are arranged two-dimensionally so as to be packed as densely as possible. Each cell 16 has a micropore 12 at the center and the micropores 12 are arranged periodically. As used herein, if the micropores 12 are arranged “periodically”, then the sum of the vectors from the geometric centroid (which will be simply referred to herein as a “centroid”) of a micropore toward the respective centroids of all of its adjacent micropores becomes equal to zero when viewed perpendicularly to the film plane. In the example illustrated in FIG. 12, the six vectors from the centroid of a micropore 12 toward those of its six adjacent micropores 12 have the same length and their directions are different from each other by 60 degrees apiece. Thus, the sum of these vectors is equal to zero. In an actual porous alumina layer, if the sum of these vectors is less than 5% of the total length of vectors, then it can be determined that the arrangement has periodicity.
The porous alumina layer 10 is formed by anodizing the surface of aluminum, and therefore, is arranged on an aluminum layer 18.
The cells 16 are formed as a result of local film dissolution and growth, which advance in parallel at the bottom of the micropores (i.e., in the so-called “barrier layer” 14). It is known that the size of each cell 16 (i.e., the pitch between two adjacent micropores 12) is approximately twice as large as the thickness of the barrier layer 14 and is almost proportional to the voltage applied during the anodization. It is also known that the diameter of the micropores 12 depends on the type, concentration, temperature and other parameters of the electrolytic solution but is normally about one-third of the size of each cell 16 (which is the longest diagonal of the cell 16 as viewed perpendicularly to the film plane).
In such porous alumina, the micropores that have been made under particular conditions have high regularity. But depending on the conditions, an arrangement of micropores with a somewhat decreased degree of regularity may also be tolerated.
Patent Document No. 6 discloses, as one of its working examples, (1) a method of forming fine unevenness on the surface of a Si wafer by dry-etching the Si wafer using an anodized porous alumina on the Si wafer as a mask. The patent document also discloses (2) a method of forming fine unevenness on the surface of an Al plate by forming anodized porous alumina on the surface of the Al plate and dry-etching metal Al using the anodized porous alumina as a mask. The document further discloses (3) a method of making unevenness on the surface of an Al plate by forming anodized porous alumina on the Al plate and dry-etching the alumina layer.                Patent Document No. 1: PCT International Application Japanese National Stage Publication No. 2001-517319        Patent Document No. 2: Japanese Patent Application Laid-Open Publication No. 2004-205990        Patent Document No. 3: Japanese Patent Application Laid-Open Publication No. 2004-287238        Patent Document No. 4: Japanese Patent Application Laid-Open Publication No. 2001-272505        Patent Document No. 5: Japanese Patent Application Laid-Open Publication No. 2002-286906        Patent Document No. 6: Japanese Patent Application Laid-Open Publication No. 2003-43203        Non-Patent Document No. 1: Masuda et al., Proceedings of the 52nd Meeting of Applied Physics Related Societies of Japan (at University of Saitama, spring 2005), 30p-ZR-9, p. 1112        
These antireflection techniques disclosed in Patent Documents Nos. 1 to 5, however, have the following drawbacks.
First of all, in the conventional uneven surface patterns, diffracted light, consisting essentially of short-wave light rays, tends to be produced at a particular angle (i.e., depending on the angle of incidence), thus decreasing the visibility eventually. Particularly when an antireflective member with such a fine uneven surface pattern is used in a display device, bluish diffracted light will be produced to decrease the visibility significantly.
Secondly, the antireflection function against regularly reflected (i.e., zero-order reflected) diffracted light is insufficient. For example, if an antireflective member is adopted in a mobile display to be used outdoors under intense sunlight, then the visibility decreases significantly. It is known that to enhance the antireflectivity, the aspect ratio of the uneven surface pattern (i.e., the ratio of the height to the period of the unevenness) may be increased. The uneven surface pattern is usually formed by a transfer process using a stamper in view of the mass productivity and other considerations. However, it is very difficult to make a stamper that will define unevenness with such a large aspect ratio. Also, even if such a stamper could be made successfully, it would be difficult to transfer the uneven surface pattern accurately enough. For that reason, if an antireflective member is made by a transfer process, the desired antireflectivity cannot be realized in many cases.
Besides, Methods Nos. (1), (2) and (3) of making a stamper as disclosed in Patent Document No. 6 use a dry process, which requires expensive equipment. In addition, since there is a restriction due to the size of the equipment, it is difficult to make a stamper with a large area or a stamper of a special shape such as a roll.
In order to overcome the problems described above, a primary feature of the example embodiment presented herein is to provide an antireflective member that can minimize the production of diffracted light, consisting essentially of short-wave light components, in a broad range of angles of incidence, can eliminate the regular reflection and can still achieve expected antireflectivity even when made by a transfer process.
Another feature of the example embodiment is to provide a method of making a stamper that can be used effectively to make a stamper with a large area or of a special shape. A more specific feature of the example embodiment is to provide a stamper that can be used effectively to make the uneven surface pattern of an antireflective member with a motheye structure and also provide a method of making such a stamper.